1. Field of the Invention
The invention relates to a stainless steel, a cold-rolled flat steel product produced from this steel, such as a steel strip or a steel sheet, and a method for producing a flat steel product from the steel in question.
2. Description of Related Art
A stainless steel which has in many cases proven successful in practice is known under the designation X5CrNi18-10 and is carried under the EN material number 1.4301. This material is a relatively soft, non-ferromagnetic austenite steel, from which, for example, pots, cutlery, wash basins, parts of domestic appliances, so-called “white goods”, such as washing machines, laundry dryers, dishwashers etc. are manufactured. According to DIN EN 10088, in addition to iron and unavoidable impurities, it typically contains (in % by weight) up to 0.07% C, 17.0-19.5% Cr, 8.0-10.5% Ni, max. 1.0% Si, max. 2.0% Mn, max. 0.045% P, max. 0.015% S and max. 0.110% N. The high nickel content here ensures the austenitic structure of the steel, which is a prerequisite for its good formability. The high Cr content here ensures good corrosion resistance of this steel.
The drawback with this steel 1.4301 is, however, that it can only be produced at comparatively high costs, as high prices have to be paid for its alloy constituents, in particular the high nickel content.
Due to the high alloying costs of the steel 1.4301, there are numerous attempts to provide a replacement for this material. The common aim of these attempts is to reduce the nickel content.
An example of a development of this type is described in EP 0 969 113 A1. The austenitic steel known from this publication, apart from iron and unavoidable impurities has (in % by weight) 0.01-0.08% C, 0.1-1% Si, 5-11% Mn, 15-17.5% Cr, 1-4% Ni, 1-4% Cu, 0.1-0.3% N, as well as relatively closely defined contents of sulphur, calcium, aluminium, phosphorus, boron and oxygen.
Another example of a steel of the type being dealt with here is known from JP 56 146862. This austenitic steel contains (in % by weight) up to 0.03% C, up to 0.5% Si, 2.2-3.0% Mn, 14-18% Cr, 6-9% Ni, up to 0.03% N, 0.15-0.50% Mo, 1-3% Cu and iron and unavoidable impurities as the remainder. In this case, particular emphasis is placed on good forming behaviour, which is adjusted by the controlled adjustment of the so-called MD30 value, which is calculated according to a special formula disclosed in JP 56 146862.
“Md30” in general designates the temperature at which after a cold forming of 30%, the conversion of austenite into martensite is 50% complete. Above this temperature, on the other hand, a reduced conversion occurs (see Werkstoffkunde Stahl, volume 2, Publisher: Verein Deutscher Eisenhüttenleute, 1985, Springer-Verlag Berlin Heidelberg New York Tokio, Verlag Stahleisen m.b.H. Düsseldorf, Chapter D 10.3.2).
The European patent specification EP 1 431 408 B1 has furthermore proposed a stainless austenitic CrNiMnCu steel with a low Ni content with the following composition (in % by weight): 0.03-0.064% C, 0.2-1.0% Si, 7.5-10.5% Mn, 14.0-16.0% Cr, 1.0-5.0% Ni, 0.04-0.25% N, 1.0-3.5% Cu, traces of molybdenum and iron and unavoidable impurities as the remainder. In order to obtain the cold-rollability, it is specified here for the δ ferrite content (“delta ferrite content”), that its content calculated by a formula disclosed in EP 1 431 408 B1 itself is less than 8.5%. The steel obtained in this manner exhibits comparable mechanical properties to the known steel 1.4301.
A stainless austenitic CrNiMnCuN steel belonging to the type of steels observed here is also known from EP 0 593 158 A1. This steel, apart from iron and unavoidable impurities, has (in % by weight)<0.15% C, <1% Si, 6.4-8.0% Mn, 16.5-17.5% Cr, 2.50-5.0% Ni, <0.2% N and 2.0-3.0% Cu. Good hot rollability, in particular the avoidance of edge cracks during hot rolling, has been achieved in this steel, at the same time as acceptable mechanical properties and corrosion-resistance. In order to reliably ensure this property combination, the Cr content of the steel is in each case adjusted here such that it certainly does not exceed 17.5% by weight.
A possibility for appropriately priced production of a steel strip or sheet consisting primarily of Mn-austenite is known from EP 1 319 091 B1, which has increased strength compared to the prior art. For this purpose, a steel is melted, which contains (in % by weight) at least the following alloy components: 15.00-24.00% Cr, 5.00-12.00% Mn, 0.10-0.60% N, 0.01-0.2% C, max. 3.00% Al and/or Si, max. 0.07% P, max. 0.05% S, max. 0.5% Nb, max. 0.5% V, max. 3.0% Ni, max. 5.0% Mo, max. 2.0% Cu as well as iron and unavoidable impurities as the remainder. A steel of this type is, in this case, cast into the casting nip formed between two rotating rollers of a twin-roller casting machine to form a thin strip with a thickness of max. 10 mm. In the meantime, the rollers or rolls are cooled so much that the thin strip in the casting nip is cooled at a cooling rate of at least 200 K/s. The known method in this manner uses the basically known technology of a strip casting system, in which it casts the steel in the casting nip formed between the rollers or rolls, for example of a two-roller casting apparatus (“double roller”) and cools it so much that a shift occurs from a primary ferritic solidification in the direction of a primary austenitic solidification. This makes it possible to transfer the nitrogen dissolved in the melt into the steel, as the austenite has high solubility with respect to nitrogen. Owing to the intensive cooling taking place at a high cooling rate, it is ensured here that nitrogen gas bubbles possibly being produced in the solidifying melt remain small and the pressure directed against them is big. This prevents emission of the high nitrogen content in the course of the solidification.
Finally, an economically producible stainless steel is known from EP 1 352 982 B1, which is also not sensitive to the production of stress cracks during conventional cold forming. In this steel, instead of the conventionally aimed for, single-phase purely austenitic microstructure, a two-phase mixed microstructure is adjusted, in which by adding Si and/or Mo and partly by lowering the Ni content or by replacing Ni by Cu, the austenite (A) and ferrite (F) proportions are adjusted. The austenite is thus stabilised to such extent that martensite formation occurring during the forming no longer leads to stress cracks. In % by weight given, the chrome content of the steel known from EP 1 352 982 B1 is between 16 and 20%, the manganese content is between 6 and 12%, the nickel content is less than or equal 9.05% and the copper content is at less than or equal to 3%. Nitrogen is to be added at between 0.1 and 0.5%. The alloy is composed such that the t-factor (ratio of ferrite-forming elements to austenite-forming elements with respective prefactors) is within a corridor of more than 1.3 to less than 1.8. At the same time, the MD30 temperature of the alloy has to satisfy a specific condition.